A theoretical model is suggested which describes the nucleation of new nanoscopic grains (nanograins) near crack tips in nanocrystalline metals and ceramics under mechanical load. In the framework of the model, nanoscopic grain boundaries split and migrate under high shear stresses near crack tips. Migration of mobile grain boundaries with disclination dipoles results in nucleation of nanograins (modeled as parallelograms in 2-dimensional nanocrystalline structures) near crack tips. This process of nanograin nucleation represents a special mode of the rotational plastic deformation and causes a partial high local stress relaxation near crack tips. The latter contributes to enhancement of the fracture toughness of nanocrystalline ceramics and metals.